dx2cg: Implement mov_sat instruction

Basically just mov but clamp the result.
https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx9-graphics-reference-asm-ps-instructions-modifiers-ps-2-0#saturate

dx2cg/disassembler.py

@@ -41,6 +41,7 @@ decls = {
 
 ops = {
     "mov": "{0} = {1};",
+    "mov_sat": "{0} = clamp({1}, 0.0, 1.0)",
     "add": "{0} = {1} + {2};",
     "mul": "{0} = {1} * {2};",
     "mad": "{0} = {1} * {2} + {3};",